Building Nuclear Weapon Would Take Years, Not Months, Iran Says in Report

WASHINGTON — The Iranian government this week published its first detailed study of how long it estimates it would take its scientists and engineers to assemble a nuclear weapon, saying that with its current infrastructure, “the required time span is in years.”

Iran described the estimate as entirely hypothetical, and it was clearly intended to allay fears that Iran has the ability to race for a bomb. Not surprisingly, American officials immediately disputed the conclusions, which contradicted both classified assessments by the United States government and many estimates by outside experts.

But the very fact that Iran’s nuclear energy establishment wrote the eight-page report, titled “How Long Would an Iranian ‘Breakout’ Really Take?” was itself notable. Until now, Iran’s public position has been that its program is entirely peaceful and that it has never studied what it would take to amass the fuel for a weapon, which is known in the nuclear world as “breakout.”

In private, according to American negotiators, the Iranians have long disputed estimates that it would take only two months or so to produce enough weapons-grade uranium, and perhaps another year, plus or minus a number of months, to fashion it into a weapon.

The report was issued as Iranian officials were announcing that they had agreed to change the design of a heavy water reactor near the Iranian city of Arak in ways that would limit Iran’s ability to forge a second path to a bomb, using plutonium. American officials said they were studying the new proposal, but saw itas a hopeful sign.

This week, two officials who last year met secretly with the Iranians to get negotiations underway — William J. Burns, the deputy secretary of state, and Jake Sullivan, Vice President Joseph R. Biden Jr.’s national security adviser — met again with their Iranian counterparts, trying to get the negotiations back on track before a July 20 deadline for a final agreement. The State Department has said almost nothing about the content of the discussion, other than that it was “constructive.”

Talks between Iran and the six nations discussing a possible deal resume next week in Vienna.

The United States argues that much of Iran’s infrastructure for producing nuclear fuel must be dismantled to significantly extend the time that Iran would need to produce a weapon. The Iranians talk of more than doubling their current number of centrifuges, which produce uranium. France’s foreign minister, Laurent Fabius, put the issue succinctly this week when he was quoted in French news reports as saying, “We say that there can be a few hundred centrifuges, but the Iranians want thousands.”

“It’s a huge gap,” one senior official involved in the talks said on Thursday, “that reflects the American insistence that we can’t live with the status quo and the Iranian insistence that they want to be able to produce all their own civilian nuclear fuel,” even for reactors Tehran is years away from building. American officials declined to be quoted by name discussing the report.

As a result, both American and Iranian officials say they believe the discussions will almost certainly go beyond the initial deadline. Under the terms of their temporary deal, which froze and rolled back some Iranian nuclear activity in return for modest relief from trade sanctions, a six-month extension of the negotiations is possible.

Iran’s report was cited in a Twitter post by the country’s chief negotiator, Javad Zarif, who said it punctured the myth about a breakout, which he said could derail any negotiation.

The report quoted the American official who Iranians say is the chief mythmaker: Secretary of State John Kerry. In April, testifying before the Senate, Mr. Kerry said, “I think it is public knowledge today that we are operating with a time period for a so-called breakout of about two months.” He said that had to be extended if the United States and its allies were to have enough warning to react.

“Six months to 12 months is — I am not saying that is what we would settle for, but even that is significantly more,” he said. That figure rattled some Israeli officials, who say they would insist on much more warning time. Israel and the United States have debated the issue for months.

“The Iranian strategy to reach a nuclear bomb is to do it not as fast as possible,” said Amos Yadlin, the former chief of Israeli military intelligence in Israel. Iran, he said, wants to do it “as safely as possible,” meaning in a way that would not provoke a military response. Mr. Yadlin is now the executive director of Israel’s Institute for National Security Studies.

The Iranians argue that Mr. Kerry is alarmist and that it will take far longer — at least 18 months, maybe 42. But the Iranian report understates the number of centrifuges Iran has in place: It notes that the country has “some 9,000,” citing reports by international inspectors. That is the number currently running, the inspection reports say, but another 10,000 have been installed.

To Americans, “breakout time” refers to the number of months Iran would need to produce enough fuel for a single weapon. Most of the Iranian report deals with the whole process of producing fuel and then making it into a weapon.

American officials note that many of those processes could happen simultaneously, reducing the amount of time needed.

A version of this article appears in print on , on Page A8 of the New York edition with the headline: Building Nuclear Weapon Would Take Years, Not Months, Iran Says in Report. Order Reprints | Today’s Paper | Subscribe